Scala example source code file (PatternExpander.scala)

This example Scala source code file (PatternExpander.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

aligned, extractor, list, nil, nopattern, notype, pattern, repeated, type, typedpat

The PatternExpander.scala Scala example source code

/* NSC -- new Scala compiler
 * Copyright 2005-2013 LAMP/EPFL
 * @author  Paul Phillips
 */

package scala
package tools
package nsc
package transform
package patmat

/** An extractor returns: F1, F2, ..., Fi, opt[Seq[E] or E*]
 *        A case matches: P1, P2, ..., Pj, opt[Seq[E]]
 *          Put together: P1/F1, P2/F2, ... Pi/Fi, Pi+1/E, Pi+2/E, ... Pj/E, opt[Seq[E]]
 *
 *  Here Pm/Fi is the last pattern to match the fixed arity section.
 *
 *    productArity: the value of i, i.e. the number of non-sequence types in the extractor
 *    nonStarArity: the value of j, i.e. the number of non-star patterns in the case definition
 *    elementArity: j - i, i.e. the number of non-star patterns which must match sequence elements
 *       starArity: 1 or 0 based on whether there is a star (sequence-absorbing) pattern
 *      totalArity: nonStarArity + starArity, i.e. the number of patterns in the case definition
 *
 *  Note that productArity is a function only of the extractor, and
 *  nonStar/star/totalArity are all functions of the patterns. The key
 *  value for aligning and typing the patterns is elementArity, as it
 *  is derived from both sets of information.
 */
trait PatternExpander[Pattern, Type] {
  /** You'll note we're not inside the cake. "Pattern" and "Type" are
   *  arbitrary types here, and NoPattern and NoType arbitrary values.
   */
  def NoPattern: Pattern
  def NoType: Type

  /** It's not optimal that we're carrying both sequence and repeated
   *  type here, but the implementation requires more unraveling before
   *  it can be avoided.
   *
   *  sequenceType is Seq[T], elementType is T, repeatedType is T*.
   */
  sealed case class Repeated(sequenceType: Type, elementType: Type, repeatedType: Type) {
    def exists = elementType != NoType

    def elementList  = if (exists) elementType :: Nil else Nil
    def sequenceList = if (exists) sequenceType :: Nil else Nil
    def repeatedList = if (exists) repeatedType :: Nil else Nil

    override def toString = s"${elementType}*"
  }
  object NoRepeated extends Repeated(NoType, NoType, NoType) {
    override def toString = "<none>"
  }

  final case class Patterns(fixed: List[Pattern], star: Pattern) {
    def hasStar      = star != NoPattern
    def starArity    = if (hasStar) 1 else 0
    def nonStarArity = fixed.length
    def totalArity   = nonStarArity + starArity
    def starPatterns = if (hasStar) star :: Nil else Nil
    def all          = fixed ::: starPatterns

    override def toString = all mkString ", "
  }

  /** An 'extractor' can be a case class or an unapply or unapplySeq method.
   *  Decoding what it is that they extract takes place before we arrive here,
   *  so that this class can concentrate only on the relationship between
   *  patterns and types.
   *
   *  In a case class, the class is the unextracted type and the fixed and
   *  repeated types are derived from its constructor parameters.
   *
   *  In an unapply, this is reversed: the parameter to the unapply is the
   *  unextracted type, and the other types are derived based on the return
   *  type of the unapply method.
   *
   *  In other words, this case class and unapply are encoded the same:
   *
   *    case class Foo(x: Int, y: Int, zs: Char*)
   *    def unapplySeq(x: Foo): Option[(Int, Int, Seq[Char])]
   *
   *  Both are Extractor(Foo, Int :: Int :: Nil, Repeated(Seq[Char], Char, Char*))
   *
   *  @param  whole     The type in its unextracted form
   *  @param  fixed     The non-sequence types which are extracted
   *  @param  repeated  The sequence type which is extracted
   */
  final case class Extractor(whole: Type, fixed: List[Type], repeated: Repeated) {
    require(whole != NoType, s"expandTypes($whole, $fixed, $repeated)")

    def productArity = fixed.length
    def hasSeq       = repeated.exists
    def elementType  = repeated.elementType
    def sequenceType = repeated.sequenceType
    def allTypes     = fixed ::: repeated.sequenceList
    def varargsTypes = fixed ::: repeated.repeatedList
    def isErroneous  = allTypes contains NoType

    private def typeStrings = fixed.map("" + _) ::: ( if (hasSeq) List("" + repeated) else Nil )

    def offeringString = if (isErroneous) "<error>" else typeStrings match {
      case Nil       => "Boolean"
      case tp :: Nil => tp
      case tps       => tps.mkString("(", ", ", ")")
    }
    override def toString = "%s => %s".format(whole, offeringString)
  }

  final case class TypedPat(pat: Pattern, tpe: Type) {
    override def toString = s"$pat: $tpe"
  }

  /** If elementArity is...
   *    0: A perfect match between extractor and the fixed patterns.
   *       If there is a star pattern it will match any sequence.
   *  > 0: There are more patterns than products. There will have to be a
   *       sequence which can populate at least <elementArity> patterns.
   *  < 0: There are more products than patterns: compile time error.
   */
  final case class Aligned(patterns: Patterns, extractor: Extractor) {
    def elementArity = patterns.nonStarArity - productArity
    def productArity = extractor.productArity
    def starArity    = patterns.starArity
    def totalArity   = patterns.totalArity

    def wholeType            = extractor.whole
    def sequenceType         = extractor.sequenceType
    def productTypes         = extractor.fixed
    def extractedTypes       = extractor.allTypes
    def typedNonStarPatterns = products ::: elements
    def typedPatterns        = typedNonStarPatterns ::: stars

    def isBool   = !isSeq && productArity == 0
    def isSingle = !isSeq && totalArity == 1
    def isStar   = patterns.hasStar
    def isSeq    = extractor.hasSeq

    private def typedAsElement(pat: Pattern)  = TypedPat(pat, extractor.elementType)
    private def typedAsSequence(pat: Pattern) = TypedPat(pat, extractor.sequenceType)
    private def productPats = patterns.fixed take productArity
    private def elementPats = patterns.fixed drop productArity
    private def products    = (productPats, productTypes).zipped map TypedPat
    private def elements    = elementPats map typedAsElement
    private def stars       = patterns.starPatterns map typedAsSequence

    override def toString = s"""
      |Aligned {
      |   patterns  $patterns
      |  extractor  $extractor
      |    arities  $productArity/$elementArity/$starArity  // product/element/star
      |      typed  ${typedPatterns mkString ", "}
      |}""".stripMargin.trim
  }
}